The present invention relates to a method for recording a halftone picture electronically, for use in a halftone picture reproducing machine for plate making.
In a previously proposed picture reproducing machine which reproduces a halftone picture electronically and directly, in order to convert picture signals having a continuous gradation into recording signals for recording a halftone picture or a halftone plate, halftone signals equivalent to those obtained by scanning a halftone contact screen are additionally generated, and then the picture signals and the halftone signals are added consecutively to obtain the recording signals for recording the halftone plate.
Concerning such halftone signal generating means, a variety of means have been proposed, and ones using a solid memory are mainly employed lately due to the increase of the capactiy and the drop of the price of the solid memory.
However, when the solid memory is used, a halftone dot corresponding to a vignette halftone dot of a halftone contact screen is quantized and stored in it. Hence, the principal nature of the halftone structure composed by the halftone signals which are generated by the solid memory, is determined in advance depending on the conditions for properly performing the quantization of the vignette half-tone dot. Such conditions are, for instance, screen angles, density steps or gradation, factors of screen pitch, and so forth.
The screen angle .theta. should satisfy a condition: tan.theta.=k/m, wherein m and k are integers, when the vignette halftone dot is quantized. The density steps are determined depending on a number S of picture elements included in one vignette half-tone dot area, as hereinafter referred to as "a halftone dot area unit", and the maximum density steps are obtained as the picture element number S=m.sup.2 +k.sup.2.
The screen pitch P corresponds to a square root of the halftone dot area unit since the halftone structure comprises a repeat pattern of the square vignette halftone dot. Thus, in the halftone structure quantized, the square root of the total picture elements included in the halftone dot area unit, i.e. .sqroot.S=.sqroot.m.sup.2 +k.sup.2, is determined as a factor .alpha.of the screen pitch P.
Meanwhile, in the color printing, in general, the halftone plates for four colors are used a set of screen angles such as 0.degree., 45.degree., 15.degree. and 75.degree., and their screen pitches P should necessarily be equal. However, when the vignette halftone dot is quantized, the proper conditions described above are given per each screen angle, and hence it is very difficult to determine the screen pitches P for all screen angles to become equal.
For example, when the condition tan.theta.=k/m for obtaining the screen angle .theta. is determined as ##EQU1## when the screen angle is 0.degree., ##EQU2## when the screen angle is 45.degree., and ##EQU3## when the screen angle is 15.degree., respective picture element numbers S.sub.0, S.sub.45 and S.sub.15 and factor .alpha..sub.0, .alpha..sub.45 and .alpha..sub.15 for screen pitches are obtained as follows. ##EQU4## With reference to the screen angle of 75.degree., it can be considered as the screen angle of -15.degree..
Accordingly, it is apparent from the above description that it is impossible to coincide the factors .alpha..sub.0, .alpha..sub.45 and .alpha..sub.15 for the respective screen pitches, which satisfy the proper conditions, even when m.sub.0, m.sub.45, m.sub.15 and k.sub.15 are selected from any integers. Hence, in practice, the integers are so selected to m.sub.0, m.sub.45, m.sub.15 and k.sub.15 that the picture element numbers S.sub.0, S.sub.45 and S.sub.15 may be similar to one another, thereby obtaining similar factors .alpha..sub.0, .alpha..sub.45 and .alpha..sub.15 of the screen pitches.
Actually, the screen pitch P is obtained by multiplying the factor .alpha. of the screen pitch to a real length l of one side of a picture element which is obtained when the quantized picture element is recorded on a photosensitive material, as P=.alpha.l.
When the vignette halftone dot is quantized and stored in the solid memory and the halftone signals are generated, the periodicity of the repeat pattern of the halftone structure is utilized. The magnitude of this periodicity affects the capacity of the memory largely.
For example, the picture element number aligning in one period is expressed by using the integer values m.sub.0, m.sub.45, m.sub.15 and k.sub.15 when the screen angle is 0.degree., 45.degree. or 15.degree., as described above, as m.sub.0, 2m.sub.45 or m.sub.15.sup.2 +k.sub.15.sup.2. Then, it is readily understood that the picture element number for the screen angle of 15.degree. is far larger than the others and thus the capacity of the memory becomes larger than the others.
Accordingly, the integer value m.sub.0 is selected so that m.sub.15.sup.2 +k.sub.15.sup.2 may be small. However, since m.sub.15.sup.2 +k.sub.15.sup.2 corresponds to the picture element number S.sub.15, if this value is determined to small, the number of the density steps is reduced.
However, the screen angles 0.degree., 15.degree. and 75.degree. are usually employed for the plates of yellow, magenta and cyan, and the number of the density steps or gradation for the yellow, magenta and cyan plates can be reduced as compared with that of a plate of black color using the screen angle of 45.degree..
Hence, the numbers of the density steps for the screen angles of 15.degree. and -15.degree. are reduced compared with that required to the screen angle of 45.degree., thereby reducing the capacity of the memory. However, as described above, if the number of the density steps is varied, the picture element numbers S for the different screen angles can not approximate, with the result of the failure of the similarity of the factors .alpha. of the screen pitches.